Bonded reinforced plastic structures

ABSTRACT

A bonded reinforced plastic structure which comprises reinforced plastic parts of sheet molding compound or bulk molding compound, coated with a primer coat and bonded together with a thermoplastic block copolymer. The primer coat comprises a moisture-curable polyisocyanate. The block copolymer comprises from 40 to 80 weight percent crystalline polyester or polyamide segments and 20 to 60 weight percent amorphous polyamide or polyether segments and has a crystalline melting point of at least 155° C. The bonded structure is adapted to survive the hot environment of paint curing ovens.

BACKGROUND OF THE INVENTION

The present invention is directed to bonded reinforced plasticstructures and more particularly to structures comprising reinforcedplastic parts bonded together with a hot melt adhesive block copolymer.

The bonding of reinforced plastics is generally accomplished withreactive multi-component thermoset systems such as peroxide curedunsaturated polyesters, polyol-polyisocyanate systems, and epoxysystems. Such systems require precise metering of the components toobtain uniform performance, they generally include toxic or sensitivecompounds which need special precautions in handling, they have a finiteopen time requiring mating of the parts to be bonded before the systemscure or advance to a nonbonding stage, their rate of cure is affected bythe moisture content of the ambient air, they require that the bondedparts be held together mechanically for rather long times until thesystems have cured sufficiently to provide a strong bond and theyrequire cleaning or purging of the application equipment to avoidsetting of the adhesives in the equipment.

Hot melt adhesives overcome such disadvantages since they are singlecomponent or single stream systems, their high molecular weight andchemical compositions make them relatively non-toxic, they are notsensitive to shock, they develop bond strength simply by being cooledbelow their softening point and do not require long periods ofmechanical fastening until cure has occurred and finally they do notpresent a problem caused by premature cure or set in the applicationequipment. However reinforced plastic structures manufactured from sheetmolding compounds or bulk molding compounds are frequently subjected toelevated temperatures such as the temperatures used in paint ovens tocure surface coatings applied to the structures. To allow the bondedstructures to survive such heat treatment, we have found that the hotmelt adhesive should be a block copolymer comprising from 40 to 80% byweight of crystalline polyester or polyamide segments and from 20 to 60%by weight of amorphous polyamide or polyether segments, the blockcopolymer having a crystalline melting point above 155° C. and a numberaverage molecular weight in the range of about 8,000 to about 30,000.Additionally we have found that to achieve adequate bond strength, thereinforced plastic parts should be coated with a thin layer of a primercoat comprising a moisture-curable polyisocyanate. The present inventionis directed to such bonded structures, to the primed reinforced plasticparts and to the method of bonding the parts by coating them with theprimer composition and applying the hot melt adhesive.

The reinforced plastic parts used in the bonded structures of thepresent invention are prepared from sheet molding compounds or bulkmolding compounds. Sheet molding compounds and bulk molding compoundsare reinforced resin systems containing about 30 to 40 weight % glassfiber, about 30 to 40 weight % inert filler such as clay, wollastoniteor calcium carbonate and about 30 to 40 wt % thermoset resin obtained byperoxide cure of unsaturated ester oligomers such as unsaturated alkydresins and ethylenically unsaturated monomers such as styrene, vinylchloride, vinyl acetate, ethyl acrylate or methyl methacrylate.

The primer coating system used in the present invention comprises asolution of a moisture-curable polyisocyanate in a low boiling organicsolvent such as a hydrocarbon or chlorinated hydrocarbon. Advantageouslysuch polyisocyanates have an average isocyanate functionality in therange of about 2.2 to about 6 and include isocyanate terminatedpolyurethane prepolymers of number average molecular weight in the rangeof about 400 to about 4000 prepared by reaction of diisocyanates such astoluene diisocyanate, 4,4'-diphenylmethane diisocyanate4,4'-dicyclohexylmethane diisocyanate, hexamethylene diisocyanate orisophorone diisocyanate with the polyols, polyester polyols or polyetherpolyols conventionally used in the manufacture of such isocyanateterminated polyurethane prepolymers; they include polymethylenepolyphenyl isocyanates; they include isocyanurate polyisocyanates ofnumber average molecular weight in the range of about 500 to about 2000obtained by oligomerization of diisocyanates or mixtures ofdiisocyanates; and they include oligomeric polyisocyanate-biurets ofnumber average molecular weight in the range of about 400 to about 2000obtained by reaction of diisocyantes with a limited amount of water. Asmall quantity of a conventional urethane catalyst can be added to theprimer coating system, such as a tertiary amine, a zinc salt such aszinc octoate or an organotin compound such as dibutyltin dilaurate. Theamount of catalyst used will be determined by the need to avoid overcureof the primer prior to application of the hot melt adhesive.

Prior to application of the primer coating, the reinforced plastic partsare advantageously cleaned with a low boiling hydrocarbon or chlorinatedhydrocarbon to remove surface contaminants. The primer coatingcomposition, as a dilute solution in hydrocarbon or chlorinatedhydrocarbon solvent, is then wiped, brushed or sprayed on the reinforcedplastic parts and allowed to dry preferably at room temperature toprovide a thin continuous coating of dry primer preferably in athickness range of about 5 to about 25 microns. The polyisocyanateconcentration of the primer solution is generally in the range of about2 to about 20 weight percent, and is preferably in the range of about 3to about 10 weight percent. Indeed the application of such dilutesolutions to the reinforced plastic parts by wiping can combine theremoval of surface contaminants and the deposition of primer into onestep. Depending upon the storage conditions, especially the ambienthumidity, the primed parts may be bonded with molten thermoplastic up toseveral days from the deposition of the primer. However, bonding is mostconveniently effected within a few minutes after application of theprimer coating.

The hot melt adhesive used for bonding the reinforced plastic parts toprovide bonds resistant to subsequent heat exposure is a block copolymercomprising from 40 to 80 weight percent of crystalline polyester orpolyamide segments and from 20 to 60 weight percent amorphous polyamideor polyether segments, the block copolymer having a crystalline meltingpoint above about 155° C. preferably in the range of from 180° to 225°C. and a number average molecular weight in the range of about 8,000 toabout 30,000. The glass transition temperature of the block copolymer,associated with the amorphous segments is generally less than 50° C.,preferably in the range of about -50° to 40° C.

The crystalline polyester or polyamide incorporated into the blockcopolymer to provide the crystalline segments, advantageously has anumber average molecular weight in the range of about 1000 to 24,000,preferably 1000 to about 6000 and a crystalline melting point of atleast about 180° C. Preferably, the melting point is in the range offrom 200° to 270° C. These hard crystalline segments contribute tensilestrength, toughness and high temperature performance to the blockcopolymer.

The crystalline polyester is a condensate of at least one aliphatic oralicyclic diol having from 2 to 10 carbon atoms and at least onearomatic dicarboxylic acid having from 8 to 20 carbon atoms, the dioland diacids being selected to provide polyesters in the desired meltingpoint range. Representative diols include ethylene glycol,tetramethylene glycol, 1,4-cyclohexane diol and 1,4-cyclohexanedimethanol. Representative diacids include terephthalic acid,isophthalic acid and 2,6-, 2,7-, 2,8-, 1,4- and 1,5-naphthalenedicarboxylic acids. The preferred crystalline polyester is polyethyleneterephthalate.

When the hard polyester segments comprise polyethylene terephthalate, asuitable molecular weight range of the polyester, prior to condensationwith the polyamide segments, corresponds to an inherent viscosity rangeof about 0.05 to about 0.7 dl g⁻¹ determined at 25° C. with a solutionof 0.5 g/100 ml in a solvent pair consisting of phenol andsym-tetrachloroethane in the weight ratio of 60:40. Preferably theinherent viscosity is in the range of about 0.1 to about 0.3.

The hard or crystalline polyamide segments of the block copolymer can becondensed from at least one aliphatic or alicyclic diamine having from 2to 12 carbon atoms and at least one aliphatic or alicyclic dicarboxylicacid having from 2 to 12 carbon atoms selected to provide a polyamidewith a melting point in the desired range. Examples of diamines includeethylene diamine, 1,3-propane diamine, 1,4-butanediamine, 1,5-pentanediamine, hexamethylene diamine, 1,10-decanediamine, cyclohexanediamine,etc. Examples of acids include oxalic, malonic, succinic, glutaric,adipic, pimelic, suberic, azelaic and sebacic acids. The hard orcrystalline polyamide segments of the block copolymer can be obtained bypolymerization of ω-aminocarboxylic acids containing from 2 to 10 carbonatoms such as aminoacetic acid, 3-aminopropionic acid, 4-aminobutyricacid, 6-aminohexoic acid, 10-aminodecanoic acid, etc. Polymerization oflactams such as ε-caprolactam provides a route to several of suchpolyamides. Among the preferred polyamides are poly(hexamethyleneadipamide) and poly(ε-caprolactam).

The soft, amorphous or low melting segments of the block copolymercontribute wettability, elasticity and rubber character to thecopolymer. They can be polyamide or polyether and are advantageously ofweight average molecular weight in the range of about 300 to about16,000 and possess a glass transition temperature less than about 50° C.and more preferably in the range of about -50° to about 40° C.

The amorphous polyamide segments can be prepared by condensing analiphatic or alicyclic diamine having from 2 to 12 carbon atoms with amixture of an aliphatic or alicyclic dicarboxylic acid having from 4 to54 carbon atoms and at least 40 weight percent of an aliphaticdicarboxylic acid having from 18 to 54 carbon atoms. The preferredamorphous polyamide segments are prepared by condensing a C₂ -C₁₀diamine with a dimer acid containing from 75 to 98 weight percent ofdimer and more preferably from 90 to 98 weight percent of dimer.Polyether segments are provided by poly(alkyleneoxy)diols such aspoly(ethyleneoxy)diols, poly(propyleneoxy)diols andpoly(tetramethyleneoxy)diols.

The block copolymers are prepared by a one step or two step method. Inthe one step method the components which form the hard or soft segmentsare polymerized in the presence of a prepolymer of the soft or hardsegments respectively. In the two step method the hard segments and softsegments are prepared separately as prepolymers and then condensedtogether.

The melting point and glass transition temperatures are convenientlydetermined with a duPont differential thermal analyzer Model DTA 900with the scanning calorimeter attachment, employing a 5 to 25 mg sampleheated at a rate of 20° C. per minute, in a nitrogen atmosphere. Themelt viscosity of the copolymer determined at a temperature of about 10°C. above the melting point of the copolymer and at a shear rate of 4sec⁻¹ is advantageously in the range of about 1000 to about 3000 poiseand is preferably in the range of about 1250 to about 2000 poise.

The most preferred group of block copolymers are blockcopoly(ester-amides) of the type described in U.S. Pat. No. 3,650,999especially block copolyesteramides of polyethylene terephthalate orpolybutylene terephthalate and polyamide of an aliphatic primary diamineand dimer acid containing 75 to 98 weight percent dimer and morepreferably 90 to 98 weight percent dimer.

In bonding two reinforced plastic parts together to form a bondedstructure, one of the primed parts is placed in a fixture adapted tohold it. Molten block copolymer is applied in a suitable pattern. Thesecond primed part is applied against the molten thermoplastic depositedon the first part and a pressure of at least about 20 kPa is appliedsubstantially uniformly over the bond, for example by clamping the partstogether in the fixture. Preferably the pressure is maintained in therange of about 28 to about 85 kPa until the thermoplastic adhesive hascooled and set. The adhesive bond line is advantageously at least about200 microns in thickness to accommodate surface unevenness and less thanabout 1500 microns to provide bonds of adequate strength at hightemperatures. Advantageously the primed reinforced plastic parts areheated to at least about 65° C. before the hot melt adhesive blockcopolymer is applied, and preferably the parts are heated to at leastabout 105° C. to increase the time that the adhesive remains in a moltenbondable state after it has been applied to the reinforced plasticparts. The primed reinforced plastic parts should, however, not bemaintained at such elevated temperatures for more than about 1 hourbefore the bonding step is carried out, to avoid overcure of the primerand adhesive failure of the bond at the primer-adhesive interface. Afterthe bond is made, the bonded structure may be cooled immediately toallow the adhesive to set or it may be subjected to an elevatedtemperature, for example in the range of 65° to 170° C., preferably 90°to 135° C. for least about 30 minutes to age the bond and to allow theprimer coat to interact with the block copolymer and cure to chemicallybond the block copolymer to the reinforced plastic parts. The agingtemperature should be at least about 10° C. below the crystallinemelting point of the block copolymer. While the molten block copolymeris generally applied from the applicator to just one of the parts to bebonded, higher bond strength can sometimes be obtained by application ofthe molten adhesive to both parts prior to mating them to form the bond.Bond strengths are determined by AST Method D-1000-72 upon 25 mm squareoverlaps of test coupons 25.4×101 mm. The bond is considered to haveattained maximum strength if failure occurs in the substrate and not atthe bond interface.

The following examples are set forth to illustrate the invention andshould not be construed as a limitation thereof. Unless otherwiseindicated, all parts and percentages are by weight.

EXAMPLE 1

Coupons (25.4×101×2.72 mm) of reinforced plastic prepared from apolyester sheet molding compound containing 33 weight percent glassfiber and 33 weight percent clay was wiped with toluene to removesurface contaminants. A 6 weight percent methylene chloride solution ofa primer composition of average isocyanate functionality of about 3,consisting of a 1:1 mixture of polymethylene polyphenyl isocyanates soldby the Upjohn Company under the tradenames PAPI 20 and PAPI 135 wasapplied to the bonding surfaces of the coupons to provide a thincontinuous film of dry primer composition.

A block copolyesteramide containing 70 weight percent polyethyleneterephthalate and 30 weight percent of a polyamide condensed from 116parts of hexamethylene diamine and 725 parts of dimer acid containing 97weight percent dimer, was prepared by condensing with the polyamide apolyethylene terephthalate of inherent viscosity 0.16 dl g⁻¹, determinedat 25° C. with a solution of 0.5 g/100 ml in a solvent pair of phenoland symmtetrachloroethane in the weight ratio of 60:40. The crystallinemelting point of the block copolyesteramide was 205° C., the inherentviscosity was 0.56 dl g⁻¹ and the melt viscosity at 216° C. and a shearrate of 4 sec.⁻¹ was 1180 poise.

The block copolyesteramide was melted in a hot melt applicator and themolten adhesive was applied to a 25.4 mm square of a primed coupon ofreinforced plastic. A second primed coupon was applied to the moltencopolyesteramide and clamped to the first coupon under a pressure of 69kPa for 2 minutes. The clamp was released. After 24 hours the bond wassubjected to a tensile shear test by AST Method D-1000-72. The bondstrength was 2510 kPa with adhesive failure at the reinforcedplastic-primer coat interface. Similar bonding tests were carried outunder various conditions of preheating of the coupons and aging of thebonds. The results are presented in Table 1. The data show that formaximum bond strength, the reinforced plastic parts should be preheatede.g. to 135° C. However when unprimed coupons were bonded under theseconditions, the tensile strength of the bond was only 1380 kPa, withadhesive failure of the bond.

When coupons were primed with an acrylic primer, a phenoxy primer, amethylated melamine, an anaerobic acrylic, a cyanoacrylate or an epoxypolyether, and were heated to 94° C. prior to bonding and aged for 45minutes at 135° C., tensile strengths in the range of 1000 to 3240 kPawere obtained with adhesive failure of the bond and no substratefailure, showing that these primers were inadequate to provide thedesired bond strength to the reinforced plastic structure.

                  TABLE 1                                                         ______________________________________                                        EFFECT OF BONDING CONDITIONS AND BOND AGING                                   ON BOND STRENGTHS OF BONDS WITH                                               POLYMETHYLENE POLYPHENYL ISOCYANATE                                           Bonding Conditions                                                            Surface                                                                              Clamp    Clamp   Bond Aging                                                                             Bond Strength                                Temp.  Pressure Time    Temp. Time Tensile                                                                             Mode of                              °C.                                                                           kPa      min     °C.                                                                          hrs  kPa   Failure                              ______________________________________                                        22     69       2       22    24   2510  10-75%                                                                        delamina-                                                                     tion                                 22     69       2       135    1   2690  25-100%                                                                       delamina-                                                                     tion                                 135    69       2       22    24   4860  100%                                                                          substrate                            ______________________________________                                    

EXAMPLE 2

Bonding tests similar to those of example 1 were carried out with anisocyanate terminated urethane prepolymer, sold by Synthetic SurfacesInc. under the tradename Tie Coat #59, as the primer. The conditions ofthe tests are set forth in Table 2. The data show that bond strength wasincreased by increasing the surface temperature of the coupons prior tobonding. Aging of the bonds was necessary to obtain substrate failure.

                  TABLE 2                                                         ______________________________________                                        EFFECT OF BONDING CONDITIONS AND BOND AGING                                   ON BOND STRENGTHS OF BONDS PRIMED WITH                                        ISOCYANATE TERMINATED URETHANE PREPOLYMER                                     Bonding Conditions                                                            Surface                                                                              Clamp    Clamp   Bond Aging                                                                             Bond Strength                                Temp.  Pressure Time    Temp. Time Tensile                                                                             Mode                                 °C.                                                                           kPa      min     °C.                                                                          hrs  kPa   of Failure                           ______________________________________                                        22     69       2       22    8    2270  Adhesive                             22     69       2       22    24   2270  100%                                                                          substrate                            65     69       2       22    1    2890  30-50%                                                                        substrate                            65     69       2       135   0.50 4000  100%                                                                          substrate                            105    69       2       22    1    3790  40-80%                                                                        substrate                            105    69       2       135   0.50 4690  100%                                                                          substrate                            ______________________________________                                    

What is claimed is:
 1. A bonded reinforced plastic structure whichcomprises reinforced plastic parts coated with a primer coat and bondedtogether by a thermoplastic block copolymer wherein the reinforcedplastic is formed from sheet molding compound or bulk molding compound,wherein the primer coat comprises a moisture-curable polyisocyanate,wherein the block copolymer comprises from 40 to 80% by weight ofcrystalline polyester or polyamide segments and from 20 to 60% by weightof amorphous polyamide or polyether segments, and wherein the blockcopolymer has a crystalline melting point above about 155° C. and anumber average molecular weight in the range of about 8000 to about30000.
 2. The bonded structure of claim 1 wherein the primer coatingthickness is in the range of 5 to about 25 microns.
 3. The bondedstructure of claim 1 wherein the moisture-curable polyisocyanate has anisocyanate functionality in the range of about 2.2 to about 6 and isselected from the group consisting of isocyanate terminated urethaneprepolymers of number average molecular weight in the range of about 400to about 4000, polymethylene polyphenyl isocyanates, isocyanuratepolyisocyanates of number average molecular weight in the range of about500 to about 2000 and oligomeric polyisocyanate-biurets of numberaverage molecular weight in the range of about 400 to about
 2000. 4. Thebonded structure of claim 1 wherein the crystalline segments of theblock copolymer are polyethylene terephthalate or polybutyleneterephthalate and the amorphous segments comprise a polyamidecondensation product of a dimer acid containing from 75 to 98 weightpercent dimer and an aliphatic primary diamine containing from 2 to 10carbon atoms.
 5. The bonded structure of claim 4 wherein the crystallinesegments of the block copolymer comprise polyethylene terephthalate. 6.A primed reinforced plastic part adapted to be bonded with a hot meltadhesive, wherein the reinforced plastic part is formed from a sheetmolding compound or a bulk molding compound and wherein the primer coatcomprises a moisture-curable polyisocyanate.
 7. A primed reinforcedplastic part according to claim 6 wherein the moisture-curablepolyisocyanate has an isocyanate functionality in the range of about 2.2to about 6 and is selected from the group consisting of isocyanateterminated urethane prepolymers of number average molecular weight inthe range of about 400 to about 4000, polymethylene polyphenylisocyanates, isocyanurate polyisocyanates of number average molecularweight in the range of about 500 to about 2000 and oligomericpolyisocyanate-biurets of number average molecular weight in the rangeof about 400 to about
 2000. 8. A primed reinforced plastic partaccording to claim 6 wherein the primer coating thickness is in therange of about 5 to 25 about microns.
 9. A process for bonding a firstreinforced plastic part to a second reinforced plastic part, whichcomprises applying a primer coating solution to the bonding surfaces,evaporating solvent from the primer coating to provide a film of dryprimer, placing the first reinforced plastic part in a fixture adaptedto hold the first reinforced plastic part, applying a moltenthermoplastic adhesive to the primed surface of the first reinforcedplastic part, mating the second reinforced plastic part to the firstreinforced plastic part to form a bond, retaining the second reinforcedplastic part in the fixture against the first reinforced plastic part byapplication of a pressure of at least 20 kPa substantially uniformlyover the bond, and cooling the bonded parts to allow the bond to set,wherein the reinforced plastic parts are prepared from sheet moldingcompound or bulk molding compound, wherein the primer coating comprisesa moisture-curable polyisocyanate, and wherein the thermoplasticadhesive is a block copolymer comprising 40 to 80 wt % crystallinepolyester or polyamide segments and from 20 to 60 wt % amorphouspolyamide or polyether segments, and has a crystalline melting point inthe range of 155° to 225° C. and a number average molecular weight inthe range of about 8000 to about
 30000. 10. The process of claim 9wherein the moisture-curable polyisocyanate has an isocyanatefunctionality in the range of about 2.2 to about 6 and is selected fromthe group consisting of isocyanate terminated urethane prepolymers ofnumber average molecular weight in the range of about 400 to about 4000,polymethylene polyphenyl isocyanates, isocyanurate polyisocyanates ofnumber average molecular weight in the range of about 500 to about 2000and oligomeric polyisocyanate-biurets of number average molecular weightin the range of about 400 to about
 2000. 11. The process of claim 9wherein the primer coating thickness is in the range of about 5 to about25 microns and wherein the thickness of the thermoplastic adhesive bondis in the range of about 200 to about 1500 microns.
 12. The process ofclaim 9 wherein the moisture-curable polyisocyanate has an isocyanatefunctionality in the range of about 2.2 to about 6 and is selected fromthe group consisting of isocyanate terminated urethane prepolymers ofnumber average molecular weight in the range of about 400 to about 4000,polymethylene polyphenyl isocyanates, isocyanurate polyisocyanates ofnumber average molecular weight in the range of about 500 to about 2000and oligomeric polyisocyanate-biurets of number average molecular weightin the range of about 400 to about 2000, wherein the crystallinesegments of the block copolymer comprise polyethylene terephthalate andthe amorphous segments comprise a polyamide condensation product of adimer acid containing from 90 to 98 weight percent dimer and analiphatic primary diamine containing from 2 to 10 carbon atoms, whereinthe primed plastic parts are preheated to at least 105° C. before themolten thermoplastic adhesive is applied, wherein the applied pressureis in the range of about 28 to about 85 kPa, and wherein the bond isaged at a temperature in the range of about 90° to about 135° C. for atleast about 30 minutes.
 13. The process of claim 9 wherein the primedreinforced plastic parts are preheated to at least 65° C. before themolten thermoplastic adhesive is applied, wherein the applied pressureis in the range of about 28 to about 85 kPa and wherein the bond is agedat a temperature in the range of about 65° to about 170° C. for at leastabout 30 minutes.
 14. The process of claim 13 wherein the preheattemperature is at least about 105° C. and the aging temperature is inthe range of about 90° to about 135° C.
 15. The process of claim 9wherein the crystalline segments of the block copolymer comprisepolyethylene terephthalate or polybutylene terephthalate and theamorphous segments comprise a polyamide condensation product of a dimeracid containing from 75 to 98 weight percent dimer and an aliphaticprimary diamine containing from 2 to 10 carbon atoms.
 16. The process ofclaim 15 wherein the crystalline segments of the block copolymercomprise polyethylene terephthalate.